Biaxially oriented polyester film has been used for various applications because of its excellent thermal properties, dimensional stability, mechanical properties, and easy control of surface morphology and, in particular, it is well-known for its usefulness as a material as a support for magnetic recording media. There have been demands for magnetic recording media suitable for higher density recording and, to further improve the high density recording capability, it is effective to further improve the smoothness of magnetic layer surfaces by using thinner magnetic layers and finer particulate magnetic material.
As a support for magnetic recording media that contains recently developed ferromagnetic hexagonal ferrite powder, there are restrictions not only on the improvement of the smooth surfaces but also on the roughening of the travelling surfaces in developing thinner magnetic layers, nonmagnetic layers, and back coat layers. Various problems can occur when magnetic recording media are stored in the form of rolls in the manufacturing process. Specifically, protrusions formed on the travelling surface are transferred to the magnetic surface to form recesses on the surface of the smooth magnetic layer, or large particles contained in the support are pushed up toward the smooth surface to generate a gentle convex undulation over the surface of the magnetic layer to decline the smoothness of the surface of the magnetic layer. If the diameter of the particles contained in the support is reduced with the aim of improving the smoothness of the surface of the magnetic layer to develop an ultrafine surface with improved smoothness, the magnetic layer will be poor in travelling property, winding property, slitting property, and surface durability.
Thus, the need to improve characteristics such as achieving traveling property and slit property together with surface smoothness can be said to always occur when developing high-density recording media.
To solve the above problems, there have been studies on the use of a polyester film that contains fine particles to control the roughness as well as the height and number of protrusions on the film surface and to depress their transfer to the magnetic layer surface (for example, Japanese Unexamined Patent Publication (Kokai) No. 2012-153100). However, even if protrusions of a specific height and size formed on the surface of the base film on the back coat layer side are defined, it still will not reduce the number of coarse protrusions or prevent reduction in the smoothness of the magnetic surface caused by their transfer when applied to a support for magnetic recording media of ferromagnetic hexagonal ferrite powder that have a thin magnetic layer and back coat layer with ultrafine surfaces. Furthermore, due to a large laminate thickness on the side where the magnetic layer is not formed, it is still impossible to eliminate the decline in the smoothness on the magnetic layer side caused by surface protrusions pushed up toward the opposite surface (the magnetic layer side). Moreover, as the height of the protrusions decreases as a result of reducing the diameter of the particles contained, the number of protrusions having a sufficient height to contribute to the travelling property decreases, allowing the problems of poor travelling property, winding property, and surface abrasion resistance to persist. In addition, there have been studies aiming to provide a polyester film whose polyester film surface undulation is controlled within a specific range to achieve both excellent winding property and electromagnetic conversion characteristics (for example, Japanese Unexamined Patent Publication (Kokai) Nos. 2012-153099, 2003-191414 and 2004-299057) or a method that applies a primer on either or both sides of a polyester film to allow the polyester film to have both good travelling property and smoothness (for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-341265). However, when applied to a support for magnetic recording media of ferromagnetic hexagonal ferrite powder requiring an ultrafine surface, production of a magnetic layer with a sufficient surface smoothness is still impossible due to particles pushed up from inside the support and transfer marks of protrusions on the back coat layer surface transferred to the magnetic layer surface. Furthermore, the travelling durability of the primer layer is insufficient and there remain the problem of in-process contamination due to dropping of particles contained.
It could therefore be helpful to provide a biaxially oriented polyester film excellent in winding property, slitting property, and dimensional stability and, when used for magnetic recording medium production, to provide a high density magnetic recording medium having a smooth magnetic layer, suffering little dimensional change due to variations in environmental parameters such as temperature and humidity or due to storage, and having good electromagnetic conversion characteristics with little dropout.